ABSTRACT Human adenoviruses (Ads) comprise more than 70 different types classified into seven species, designated A to G. Ad infections cause diseases of the respiratory track (species B, C, and E), eyes (species D), and gastrointestinal (GI) track (species F). Adenovirus type 40 (Ad40) and Ad41, the only two types making up species F, are one of the main causative agents of acute gastroenteritis in children worldwide. To date, little progress has been made in our understanding of species F pathogenicity, in marked contrast to Ads of other species. The main goal of this application is to fill our gap in knowledge on immune evasion mechanisms of the enteric species F Ads. The early transcription unit 3 (E3) of Ads encodes proteins that interfere with the human immune system, and, notably, it is one of the most divergent gene regions between species. This suggests a role for E3 immunomodulatory proteins in the species-specific disease patterns exhibited by Ads. Interestingly, in comparison to other Ad species, the E3 region of species F contains a unique set of protein-coding genes whose functions are unknown such that, to date, no immune evasion functions have been uncovered for this species. The objective of this grant is to determine the mechanism employed by Ad40 and Ad41 of species F to modulate host immune functions and identify the viral protein responsible for these effects. The hypothesis is that an E3 protein, unique to Ad40 and Ad41, binds to the MHC class I-related chain (MIC) A and B proteins inside infected cells of the GI track as a strategy to compromise natural killer (NK) cells. The cell surface recognition of MIC A and B ligands by the NK group 2 member D (NKG2D) activating receptor on NK cells is critical for detection and elimination of virus-infected cells by the host. Because MIC A and B are constitutively expressed only on intestinal epithelial cells, and because Ads of species F show a narrow tissue tropism for the GI track, we rationalize that these viruses must have evolved to cope with the immune system of the gut by expressing an immunoevasin that downregulates the cell surface expression of MIC A and B. We have obtained preliminary data in Ad41-infected human cells in support of our hypothesis. In this proposal, we will use a functional (Aim #1) and biochemical (Aim #2) approach to test our hypothesis by studying the effects of Ad40 and Ad41 infection on expression of various cell surface immune proteins, identify the E3 protein that mediates these effects, and determine if this protein can directly associate with the MIC proteins. The significance of our studies is highlighted by the clinical threat that species F Ad infection presents in children worldwide and the current lack of effective treatments.